1. Field of the Invention
This invention relates to cathode structures for cathode ray tubes, and more particularly relates to a method for producing such structures from slurry compositions of cathode material containing urea.
2. Prior Art
Cathode structures for cathode ray tubes desirably exhibit uniform electron emissions over an extended life cycle and under a variety of operating conditions. In addition, such cathode structures must be manufactured at the lowest possible cost. Because of such stringent requirements, particularly reliability and cost, there is great reluctance on the part of high volume manufacturers of cathode ray tubes to introduce new cathode structures or methods. Nevertheless, presently used cathode structures and methods exhibit limitations sufficiently troublesome to justify continuing investigations of alternate structures and methods.
One such limitation is poor adherence of the emissive layer to its substrate.
Adherence problems arise, particularly during operation near the high end of the normal temperature range, and can appear as lifting, flaking, or blistering of the emissive coating. Such adherence problems may be due in part to incomplete contact between relatively porous sprayed coatings and the underlying substrate. Such poor adherence can contribute to lower emmision and shorter life of the cathode.
Slurry cathode coatings are known, having been described in U.S. Pat. No. 4,170,811. Therein, accurate alignment of the cathode coating with the electron gun apertures is achieved by inserting a dispensing tube through the grid apertures and depositing a slurry droplet on a supporting substrate much larger than the droplet, and allowing the droplet to spread and dry in an uncontrolled manner. Because the grid and substrate have already been assembled prior to slurry application, and because there is little control over the thickness of the cathode layer, there is also little control over the K-G.sub.1 spacing and cut-off voltage.
In co-pending U.S. Patent Application Ser. No. 335,300, referred to above, cathode structures are produced by dispensing drops of a slurry of controlled viscosity and concentration of cathode particles onto a substrate, and drying the drop without spreading to achieve a cathode layer of controlled density, thickness, and surface smoothness. Such structures exhibit good adherence between the cathode layer and substrate and uniformity of electron emissions, as well as enabling close control over K-G.sub.1 spacings in electron gun structures for cathode ray tubes. However, in certain instances too rapid removal of organic binder material from the relatively dense dried cathode layer can weaken the layer or otherwise deleteriously affect its adherence to the underlying substrate.
Accordingly, objectives of the present invention include: providing an improved slurry method for producing cathode structures for cathode ray tubes whose cathode layers exhibit improved adherence to their supporting substrates.